X Forwarding over SSH

Hi all, I've been trying to get this working for several days and finally figure that I need some help. I have an Arch box that I use for backups and for compiling large C programs that I recently installed all the X stuff on so that I could get some graphical apps on my Windows box. I thought the setup should be easy, with sshd_config:

X11Forwarding yes
X11DisplayOffset 10
X11UseLocalhost yes

and ssh_config

ForwardX11 yes

I use putty on the Windows machine with X11 forwarding enabled at localhost:10.0, with the Cygwin Xserver running. When I try to run an X app, xclock for instance, I get:

Error: Can't open display: localhost:10.0

I'm not totally sure on the numbers after the 'localhost:', they're just my best guesses. Any ideas?